CALCULATED SITE SUBSTITUTION IN TERNARY GAMMA'-NI3AL - TEMPERATURE AND COMPOSITION EFFECTS

The temperature and composition dependence of the site substitution be havior of ternary additions to Ni3Al is examined on the basis of first -principles calculations of the total energies of ternary, partially o rdered (gamma') alloys. The calculations are performed by means of the linear muffin-tin orbitals method in conjunction with the local-densi ty and multisublattice coherent-potential approximations and include a ll 3d, 4d, 5d, and noble metals. The calculations show the existence o f simple trends in the alloying behavior of the gamma' phase which may be explained in a Friedel-like model based on the interaction between Ni and the added species. It is shown that the commonly accepted inte rpretation of the site substitution behavior of Cu and Pd may be incor rect because of site substitution reversal at high temperatures. It is further shown that the direction of the solubility lobe in the ternar y phase diagram for the elements Co, Pd, Cu, and Ag incorrectly has be en interpreted as evidence for strong Ni site preference and that, in fact, these elements are expected to exhibit only weak Ni site prefere nce.